The term "portland cement" refers to a hydraulic cement (one which sets hard under water) obtained by burning a mixture of lime and clay to form a clinker which is pulverized to a greenish gray powder. Portland cement is composed primarily of hydraulic calcium silicates, with lesser amounts of calcium aluminates, calcium aluminoferrite, and one or more of the forms of calcium sulfate as an interground addition. When mixed with water, portland cement solidifies to an artificial rock, similar to portland stone.
The term "concrete" refers to a composite material primarily composed of a binding medium within which are embedded particles or fragments of aggregate having a wide range in mineralogical composition and fineness distribution. In portland cement concrete, the binder is a mixture of portland cement and water. The aggregates usually consist of a fine fraction, such as a natural sand, and a coarse fraction, such as a river gravel or a crushed stone. Mortar contains only the fine sand aggregate with water and cement paste.
In cold weather, e.g., 35.degree.-50.degree. F. (1.5.degree.-10.0.degree. C.), portland cement can have an extremely long period, e.g., 8 to 14 hours or longer, prior to the start of initial hardening, whereas under normal temperatures, 70.degree.-75.degree. F. (20.degree.-24.degree. C.), initial hardening occurs at about 21/2 to 4 hours.
Calcium chloride (CaCl.sub.2) is a commonly and widely used accelerator for portland cement concrete in cold weather to provide for acceleration of set. However, chloride-containing compounds have been shown to lead to the instigation of galvanic corrosion of steel rebar in reinforced concrete. Also in the presence of dissimilar metals in concrete, such as steel reinforcement, aluminum conduit, and galvanized metal decks, the potential for corrosion is magnified further when chloride-containing compounds are in excess of about 0.15% chloride ion by weight of portland cement. Thus, the need for an accelerator which is non-corrosive and non-chloride-containing is evident.
Calcium nitrate Ca(NO.sub.3).sub.2 by itself, as well as in mixture with amounts of other compounds, has been used in the concrete industry to accelerate the setting time of portland cement concrete in cold weather. These calcium nitrate products are used in liquid form. Generally, the calcium nitrate used as an additive and accelerator with portland cement is a relatively high grade material having only minor amounts of impurities.
One publication which discloses the acceleration of concrete setting with calcium nitrate is Kantro, D. L., "Tricalcium Silicate Hydration in the Presence of Various Salts," Journal of Testing and Evaluation, JTEVA, Vol. 3, No. 4, July, 1975, pp. 312-321. Often, the calcium nitrate is mixed with other ingredients, such as alkanolamines or urea.
U.S. Pat. No. 4,337,094 discloses an additive composition comprising a concentrated aqueous solution of calcium nitrate, Ca(NO.sub.3).sub.2, in combination with polyalkanolamines, preferably such as obtained in a triethanolamine (TEA) bottoms waste stream, when added to a portland cement-containing composition, provides good set acceleration and compressive strength enhancement. The patent states that the additive composition can provide acceleration in a chloride-free environment.
U.S. Pat. No. 4,606,770 discloses a hydraulic cement mix including hydraulic cement aggregate, sufficient water to affect hydraulic setting of the cement, and an additive comprising an N-methylol amide, specifically, tri- or tetramethylolglycoluril (herein referred to as TMGU) alone or in combination with other accelerators of set, such as calcium nitrate, to decrease the time necessary for hardening of the mix. However, the hydraulic cement mix produced by the process of U.S. Pat. No. 4,606,770 is understood to evolve copious amounts of ammonia in the fresh plastic state of the concrete, due to the presence of TMGU which, when mixed with concrete, breaks down and forms urea, which further breaks down to release ammonia in the fresh concrete.
U.S. Pat. No. 4,064,191 discloses an organic solvent solution of a mixture or blend of certain partially or fully alkylated glycoluril derivatives and certain organic solvent-soluble, normally non-self-crosslinking polymeric materials that are acid catalyzed which can be used as a coating resin on metal substrates.
Heretofore, a relatively inexpensive calcium ammonium nitrate hydrate composition of the general formulation Ca(NO.sub.3).sub.2 NH.sub.4 NO.sub.3 10 H.sub.2 O has been commercially available and used as fertilizer. One such product identified as industrial grade calcium nitrate is supplied by Norsk Hydro, a.s. of Norway and sold in the United States by William and George Meyers Co. of Toledo, Ohio. However, such a product was not used effectively as an accelerator for portland cement because when mixed with portland cement and water, the initial pH of the mixture is so high (greater than 12) as to cause evolution of ammonia gas from the composition during the mixing and placing of concrete, resulting in a health hazard or at least causing irritating conditions. West German Offenlegungsschrift No. DE-3543874-AI, inventor Hans-Rudiger Braun, which was published June 19, 1987, discloses the use of formaldehyde as a crosslinking agent to convert the ammonia which is present in the calcium ammonium nitrate hydrate type fertilizer composition to form a mixture of calcium nitrate, sodium nitrate, hexylmethylenetriamine, sodium hydroxide, and water. The composition is stated to be useful as an accelerator in portland cement.